A quadrupole-orthogonal time-of-flight (TOF) mass spectrometer typically operates in a mode in which ions generated from an ion source pass through a series of vacuum ports and ion guiding devices, and enter a quadrupole mass analyzer for mass selection. The selected parent ions enter a collision cell and are disassociated, to produce many daughter ions. The daughter ions enter a pulsed acceleration region before a flight chamber, and are orthogonally accelerated. Due to different flight times of the ions, a high-resolution and high-precision mass spectrum is generated. Wherein, the quadrupole mass spectrometer generally continuously operates in a scan mode, and the TOF mass spectrometer operates in a pulse mode. If the ions before the TOF mass spectrometer are not modulated in any way, for the pulse voltage in the acceleration region before the flight chamber, a next pulse can be generated only after the ions with the largest m/z ratios reach the detector. However, the ions enter the acceleration region continuously. As a result, the duty cycle that the ions are used by the TOF mass spectrometer is too low, thus causing the ion loss. If the distance from an electrode in the acceleration region to the detector is D, and an effective width of the electrode in the acceleration region is Δl (which may be deemed as a width of the ion beam that is accelerated before the acceleration region and forms a mass spectrum finally on the detector, and generally smaller than the actual width of the acceleration electrode), the maximum ion utilization efficiency (or referred to as duty cycle) of the instrument is related with m/z ratio of the ions:
                              Duty          ⁢                                          ⁢                      cycle            ⁡                          (                              m                2                            )                                      =                                            Δ              ⁢                                                          ⁢              1                        D                    ⁢                                                    m                ⁢                                  /                                ⁢                z                                                              (                                      m                    ⁢                                          /                                        ⁢                    z                                    )                                max                                                                        (        1        )            
where (m/z)max is an upper limit of the mass range. In most orthogonal TOF mass spectrometers, the duty cycle ranges from about 5% to 30%. If an ion gate or ion trap is used, although the ions can impulsively enter the pulsed acceleration region before the TOF mass spectrometer, the ions experience a flight process before entering the acceleration region, therefore the ions of different m/z ratios are broadly distributed, and only ions of a certain range of m/z ratios can reach the acceleration region substantially at the same time. Therefore, the mass range is greatly limited.
Efforts are made to try to solve the problem in the prior art. For example, in U.S. Pat. No. 6,770,872 or 7,208,726, a three-dimensional ion trap is positioned before the TOF acceleration region, such that the ion trap and the TOF mass spectrometer operate cooperatively. In U.S. Pat. No. 7,714,279, a radio-frequency guiding device is used to store and release ions, ions with a small m/z ratio are released initially, and the pulse acceleration voltage is synchronized with the released ions by adjusting the parameters of a following device. In Patent No. WO2007/125354, a radio-frequency potential barrier is formed in a stacked-ring electrode array arranged along an axial direction, and the sequential release of ions according to the m/z ratios can be achieved by changing the balance between a traveling wave voltage or DC voltage along the axial direction and the radio-frequency potential barrier. In U.S. Pat. Nos. 7,208,728 and 7,329,862, two linear ion traps are disposed along the axial direction, one is for resonant excitation in the axial direction to selectively eject ions out, and the other is only for synchronization with a pulse acceleration voltage, rather than for mass selection. In this way, a duty cycle of more than 60% is obtained. The most effective and simple solution at present may be a device called “Zeno trap” proposed in U.S. Pat. No. 7,456,388, in which ions are sequentially ejected in an order of m/z ratios from largest to smallest by shifting the balance between the radio-frequency potential barrier and the DC potential barrier at the end of the device in an axial direction. The released ions are accelerated along the axial direction to have a low energy (20-50 eV), ions with a large m/z ratio have a low speed, and thus are gradually caught up by ions with a small m/z. By adjusting the speed of the released ions, ions of different m/z ratios can reach the acceleration region before the flight chamber substantially at the same time. In this manner, a duty cycle of nearly 100% can be obtained.
However, the above solutions still have problems. For example, as is known to those skilled in the art, for the Zeno trap, after the ions are released along the axial direction by overcoming the potential barrier, a long period of time is needed to cool in the radial direction, or otherwise, it is difficult to attain a high resolution of the TOF mass spectrometer. Therefore, the scanning frequency of the Zeno trap is generally about 1 kHz, which is much slower than a common pulse acceleration frequency (5-10 kHz). Accordingly, a quite high storage capacity is needed for obtaining a high ion utilization efficiency at a low scanning speed. However, the storage capacity of the Zeno trap is not higher than that of a common linear ion trap, that is, not higher than an order of magnitude of 105. As such, the dynamic range of the instrument is heavily limited. The ion storage capacity can be enhanced to some extent by extending the length of the Zeno trap. However, this will lead to a bulky instrument on one hand, and a large amount of ions are broadly distributed in the axial direction on the other hand. Therefore, an extended period of time is needed for release, whereby the scanning speed of the instrument is further reduced.
Therefore, there is a need for an improved technical solution to solve the above problems.